Ultrasound-assisted freeze-drying process for polyimide aerogels

Owing to the unique structural characteristics, aerogel has broad utilization prospects in numerous application scenarios such as thermal insulation, electromagnetic interference shielding, adsorption, etc. As one of the most commonly used drying methods in aerogel preparation, freeze-drying avoids surface tension at the gas–liquid interface by constructing a gas–solid interface. However, problems such as relatively low freezing efficiency, aerogel structure destruction, excessive and uncontrollable pore size caused by uncontrolled growth of crystals during freezing process still severely limit the preparation efficiency and performance of aerogel. In this work, the reported ultrasound-assisted freeze-drying (UAFD) strategy was applied to enhanced the freezing efficiency by cavitation-induced nucleation. Meanwhile, by affecting the secondary nucleation, UAFD strategy produced aerogel with smaller pore size and narrower pore size distribution. At the same time, the pore structure could be tuned by the regulation of ultrasonic power. The thermal insulation, hydrophobicity of polyimide (PI) aerogels and electromagnetic interference (EMI) shielding properties of PI-derived carbon aerogels prepared by UAFD strategy were significantly improved. The strategy proposed in this work provides a new path to achieve improvement of aerogel production efficiency and performance through structural regulation. Moreover, it could be extended to the preparation of other types of aerogels.